1. Field of the Invention
One of the problems associated with holograms is that diffusion type holograms which provide holographic pictures of objects, for example, are degraded by the effects of the spurious reflection and transmission hologram recordings that are generated during the holographic exposure process. These spurious holograms have been found to be generated by reflections from surfaces which are interfaces of materials of differing index of refraction such as air to glass interfaces of the transparent surfaces of the recording cover plates, the substrate, the gelatin, the recording medium or the optical elements. These reflections combine with the main holographic beams and with each other to form both spurious reflection hologram recordings and spurious transmission hologram recordings on the recording film. In a hologram display, the spurious reflection hologram recordings create ghost images and the spurious transmission hologram recordings create rainbow-like flare patterns. The purpose of this invention is to prevent the recording of the unwanted spurious holograms that are caused by reflections and thus to eliminate both ghost and flare images in the final hologram recording. Diffusion holograms are used to form data holograms, directional projection screens (either front or rear projection screens), jewelry holograms, art holograms and other pictorial holograms, all of which conventionally have undesired characteristics caused by the presence of spurious hologram recordings.
2. Description of the Prior Art
Prior to this invention there was no known method to reduce flare and ghost images of diffusion type holograms to acceptable levels. The basic approaches that have been utilized in an attempt to minimize the problem are to anti-reflection coat all reflection surfaces of the hologram forming system, to embed all optical elements in an index matching fluid when forming the hologram such that no interface reflections reach the hologram area, or to form the hologram with the beams impinging the film supporting elements at the Brewster angle. The first method has been found to be inadequate because the best known anti-reflection coating provides results that are much worse than required. The second method is generally unsuccessful because the striations in large fluid volumes degrade the holographic recording process. The third method is unsatisfactory because of the restriction of the angular placement of the recording, and hence playback, beams to the recording medium.